If you’re interested in reducing a building’s energy use without affecting comfort, then thermodynamics is for you. Thermodynamics is all about the relationship between temperature and energy. Using the principles of thermodynamics, we can keep people comfortable in a building while using energy in smarter ways.This is because thermodynamics deals with how heat and temperature are related to energy. The word comes from ancient Greek roots “thermo” meaning hot and “dynamis” meaning power, but the modern science of it is only two centuries old, and in some ways even newer than that.Within a building, all of the heating and cooling systems and equipment are governed by the laws of thermodynamics. The interactions between the systems are also governed by these laws.You may have heard of the first law before: energy can neither be created nor destroyed.You might think that this means there is no way to be efficient, because energy is getting used up one way or another to make a room cold. But you’d be wrong. Thermodynamics teaches us numerous tradeoffs and rules that allow us to get the right temperature in the right place at the right time for the least amount of energy.The details of how to make these tradeoffs work are actually maddeningly complex, not least of all because different engineers and physicists can’t agree on what set of measurements to use. We can analyze the journey of air molecules in a building through so many tools: what temperature it is, what pressure it is, what the entropy is, what volume it occupies. These are all related and interchangeable measurements, but different experts use different ones and feel strongly about it. It’s as if one person is talking about how far away a store is by saying “It’s ten minutes away” and another says “It’s two miles away” and they argue past each other.As air molecules journey through the building, one key to minimizing energy use is to make sure that when heat is transferred it’s done with large temperature changes. You can imagine a fan that pushes a lot of cold air through a building. If that air isn’t warming up very much because the building just doesn’t need very much cold air, you can bet you are wasting energy pushing all that cold air around. On the other hand, if you push a lot of cold air through a building and it comes back warm, then you’re achieving a big temperature difference – a big dT (“dee tee”) in the parlance of engineers – and you’re doing things more efficiently.The refrigeration cycle, latent heat, and the importance of compressors, are just a few of countless other aspects of thermodynamics that are important for building.The key idea to remember is that in all that big metal equipment, in all those fans and pumps and ducts and pipes, the air particles that are keeping us comfortable are bouncing around and doing some pretty fascinating things. Those things are governed by the laws of thermodynamics, and the better we understand those laws, the more calm and comfortable we can be while using less energy.